Industrial robots are well known with articulated arms which comprise a lower arm provided on the robot body pivotably about a first horizontal axis, a forearm provided on the lower arm pivotably about a second horizontal axis, and a wrist assembly provided on the forearm. Generally, in industrial robots of this type, the robot body has a forearm drive motor provided thereon. The revolution of the output shaft of the motor is transmitted to a first sprocket fixed to the lower arm coaxially with the first axis, and the revolution of the first sprocket is conveyed by means of a chain to a second sprocket fixed to the forearm coaxially with the second horizontal axis.
In robots with articulated arms of the above-mentioned type, the movement of the forearm itself normally ranges from a generally horizontal position or a position where it is slightly tilted upward toward the upper end thereof, to a position at which the upper end is directed nearly normally downward. In robots of this type, a moment yielded around the first axis due to the weight of the forearm and wrist mechanism is applied to the forearm drive motor by means of a revolution transmitting mechanism; however, since this moment due to the weight varies as the forearm is tilted, the drive torque of the forearm drive motor will vary according to the tilt angle of the forearm. Normally, when the forearm is in the normal or standing position, the moment due to the weight is at its largest. As the upper end of the forearm is tilted downward, the moment decreases along a cos curve. When the upper end of the forearm is directed normally downward, the moment due to the weight is zero. To provide a moment opposite to such a moment, a forearm weight balancing mechanism is incorporated in the conventional robots. FIG. 4 schematically illustrates the structure of a prior-art forearm weight balancing mechanism. As seen in FIG. 4, the movable body has provided thereon a rotating disk 2 interlocked with a forearm 1 and rotatable about the first axis C1. A tension spring assembly 3 has one end 3a thereof coupled to the robot body (not shown) and the other end 3b coupled to the rotating disk 2.
The forearm weight balancing mechanism of the above-mentioned type works as follows: Against the moment M yielded around the second axis C2 due to the weight acting on the forearm 1, an opposite moment -M is provided around the first axis C1 by the resilient force of the tension spring assembly 3; however, since the pulling force of the tension spring assembly 3 increases as the forearm 1 is tilted downward from the normal or standing position, the opposite moment -M due to the tension spring assembly 3 increases as the forearm 1 is tilted normally downward so that it will not be in balance with the moment M due to the weight.